A hatch back type back door has been used widely for not only vans and wagons but also compact passenger cars because the back door of this type can open a vehicle body rear part widely and can be kept in a jumped-up state, and also access to a passenger room (cargo room) space is easy (refer to JP2005-193827A). For a vehicle equipped with such a back door, it has been known that booming noise is generated at the time of low-speed running. When vibrations of an engine rotating at a low speed close to the resonance frequency of back door are transmitted to the vehicle body rear part, the back door vibrates in the vehicle longitudinal direction, and periodic changes in volume take place in the vehicle compartment, whereby booming noise is generated.
As the above-described vibrations of back door, in addition to the vibrations the center of which is a back door hinge, which are caused by elastic deformation of a weather-strip and a stopper, flexural vibrations of the back door itself are assumed. Therefore, to reduce the booming noise, in addition to the improvement in back door mounting rigidity, effective improvement in rigidity of the back door itself is demanded. This demand conflicts with the reduction in weight of the vehicle body. For a compact car in which emphasis is placed on fuel economy and manufacturing cost, it is difficult to achieve a remarkable effect of reducing booming noise in the allowable range in comprehensive performance evaluation.